1. Field of the Invention
The invention relates to the production of Si and somewhat more particularly to a process of manufacturing Si for semi-conductor components, such as solar cells, whereby quartz sand is utilized as the initial raw material and the Si is obtained through reduction of SiO.sub.2 in an electrical arc.
2. Prior Art
The technical fields of application for Si known in semiconductor technology is limited by the degree of purity of the raw or source material thereof. As is known, so-called technical-grade silicon (having a purity of about 98%) is manufactured from quartz sand and carbon in an electrical arc and contains as major impurities, iron, boron and phosphorus. Boron and phosphorus have particularly detrimental effects on any semiconductor components produced from such silicon because these elements decisively affect the conductivity characteristics of the semiconductor material.
Therefore, in order to produce highly purified Si for semiconductor purposes, through reduction of SiO.sub.2 in an electrical arc, quartz sand with a low impurity level is required as the raw or source material. However, quartz sand with the necessary degree of purity is available world-wide only to a limited extent and is thus very expensive. On the other hand, if one utilizes as the Si source material, ordinary quartz sand which has been systematically mined, the demanded degree of purity cannot be obtained through the electrical arc reduction process.
German Offenlegungsschrift No. 26 23 413, which is essentially equivalent to U.S. Pat. No. 4,097,584, describes a method for manufacturing highly purified silicon for semiconductor components whereby, after the reduction of SiO.sub.2 with carbon in an electrical arc, the silicon, which still contains impurities (98% purity) is treated in its moltent state in the presence of water, with hydrogen or hydrogen-containing compounds so that phosphorus present in such silicon escapes therefrom in the form of phosphorus hydride (or hydrogen phosphite) whereas boron remains in the form of boron oxide in the slag on top of the moltent silicon and can be readily separated therefrom. This procedure is preferably conducted in a reactor at sub-atmospheric pressures. The so-obtained silicon has an impurity level of less than about 1 ppm of boron and phosphorus therein.